Communication between ships



A118 27 1946- w. B. ELLwooD COMMUNICATION BETwEENsHIPs Filed oct. 4, 1944 /NVENTOR WB. ELLWOOD er Afro/MEV Patented Aug. 27, v1946 COMMUNICATION BETWEEN sHIPs Walter. Ellwood,r New York, N. Y.,

assgnor to Bell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York v Application October 4, 1944,`Serial No. 557,194

(ci. 17a-t2) 8 Claims.

This invention relates to communication systems and more particularly communication systems employing low frequency electromagnetic fields.

An object of this invention is to provide an improved communication system employing low or audio frequency alternating magnetic fields.

In accordance with the present invention an improved receiving device is provided capable of responding to extremely Weak alternating current electromagnetic fields in the audio Vfrequency range and giving an amplified output. I

In accordance with an exemplary embodiment of the present invention an elongated core o-f magnetic material is placed in the electromagnetic field. The core has a center section of suitable magnetic material which is provided with windings for producing voltages in accordance with the electromagneticfeld. In order to improve the sensitivity of the device ,a source of alternating current is applied to certain of the windings and the output voltage derived from other of the windings.

Magnetic detectors and` amplifiers provided with somewhat similar cores and windings have been employed in the past to detect and produce amplified voltages in response to electromagnetic fields. In the devices employed for thispurpose in the past, the alternating current or the, alternating current field has been sufficient to, or exceeded, that required for saturation of the magnetic material. However; the devices of the lprior art are not suitable rfor detecting weak fields; first, because they are not sensitive to weak elds;

and second, because they generate. appreciable i noise which tends to mask or interfere with the received signals. 1

In accordance with the present invention .the

alternating current supplied to the. device does not tend to saturate the magnetic material of the cores employed. Instead the magnitude of the alternating current is maintained suiciently small so. that the ilux in the material does not saturate the material or even approachthe socalled knee of the magnetization curve. 'I'hel magnetic material is operated exclusively in the low iiux density region where the magnetization curve or the hysteresis loop is approximately defined by Rayleighs law. The manners and modes of operation-.of magnetic material'` inthe low' flux reg-ion is described in greater detail in La paper kry E.v Peterson entitled HarmonicProyductiomin ferromagnetic materials at` low-frequencies an'd low yflux densities'and published inthe Bell' System Technical* Journal for October 1928, vol. 7, pp, 762-796. See also Magnetic hysteresis at low flux densities? by Ellwood, published in Physics for July 19,35, vol. 6, pp. 215-226, and Study of magnetic losses at low flux densities in permalloy sheets byEllwood and Legg, published in the Journal of Applied Physics for May 1937, V01. 8, DD. 313-358.

One possible explanation for theimproved results obtained at low elds isY that the Barkf hausen steps are much smaller at low iields than at high elds. Consequently, the lmaterial responds to smaller changesin' the applied field. Furthermore, since the steps are smaller the amplitude of the noise dueto this-effect is smaller.

When-a magnetic device in accordance with this invention is operated in this region it becomes extremely sensitiveto small applied fields, l

produces a ,considerably greater voltage output for said small iields, and produces less noise than is produced by the magnetic devices of the prior art. n l

.I-Ioweven since it is necessary to operate the material at extremely low valuesofapplied field and flux, it is necessary to accurately. control the bias or operating point on the magnetization curve and to balance out 'the core flux from sources other than thesourcefrom which it isdesired to receive signals. Devices of this-type :are capable of responding to electromagnetic iields considerably weakerthan the earths magnetic field. Consequently, it is also necessary to balance out the ux of the earths magnetic field from the core or cores of the present device. In order to provide the necessary balancing currents or voltages, the output of the device is rampliiied and the amplified components of ythe output having frequencies below the signalingy frequency, including direct current, are employed to neutralize the earths field and other stray low frequency fields having frequencies belowf the signaling `frequency. The foregoing objects and features and other objects and features of the present invention may be more fullyunderstood from the following description of an exemplary embodimentvthereof when said description is readwith referenceto Y the drawing-which shows in schematic form the .which` speciiicallv` point various elements' which cooperate to provide an improved receivingdevice for receiving -audio frequency electromagnetic waves. The following description is not in anyway intended to limit or increase the scope of the claims appended hereto out the novel features of this invention. y f l Referring now to the drawing l and 2 repreleast two legs 4 and made of magnetic material which possesses eitherl .meabilities of sections I,

rappears across the output vterminals sent an elongated core of magnetic material.

This core is placed in the region where the mag-1 netic field is to be detected. For the purpose of illustration, it is assumed that it is desired to receive an electromagnetic field of 25 cycles. It will be apparent to persons skilled in the art that the invention is not limited to any particular` frequency or frequency range but may be employed'in magnetic fields of any suitable frequency. Inasmuch as such low frequency fields,Y

are frequently employedl in communication between ships the core will be madeY relatively long y and mounted aboard ship preferably above deck and away from the major portionV of the magnetic material and structure of the ship, as for example, to 15 feet above the Ideck of the ship.' When desired this coreand coil structure vmaybe made in the form of a cable and towed by the ship. It may be necessary to float the device inquiet water if extreme range is desired.

The core is'composed ofv a plurality-of sections 'I,v 2.' and 3, as lshowni'n Fig. 1; These core sections maybe made of any suitable magnetic material as, for example; any of the various iron-nickel alloys-which have a high'permeability at low Imagnetic elds.

The core sections I and 2 are separated by center section 3 comprising a magnetic core having at 5.l The center section 3 is a'high hysteresis coeicient or a high rate of change of permeability'witn field at low values of applied field or flux or both. A particularly Suitable type of core material is' describedin-detail in the United States Patent 1,768,443 granted toY -Elmen June 24, 1930, which patent is hereby made part of the present application by reference as if fully' included herein. Howevenit can be shown that-'the device is most effective when the per- 2 and 3 are high and approximately equal while the material of the .center section 3should have either as high a hyshigh .a rate of changeV teresis coeiiicient orf as of permeability with eld as possible or both.

Each of the legs 4 and having a frequency which is high compared to the highest signaling'frequency or vcomponent thereof 'to be transmitted while windings 8 and 9 are connected to the receiving system as will-bel described hereinafter.'

The windings B, 1, 8 and 9 are so connected relative to each other that when no external mag-- l netic eld is applied to the composite core struc- 2 and 3no voltage 23 and v24 windings 6 and 'I ture comprising sections I,

of coils 8 and 9 even when the are connected to source I0. However, when an external field is applied tothe core structure comprising elements I, Zand 3.a voltage is produced -between terminals 23` and 24. This output voltage maybe due to either theY hysteresis of the core material or due tothe change in the permeability of the core material orY both as described in the above-identified publication by Peterson. All. of the above-identified publications are hereby made a part of ythis application as ifA fully included herein. f

For amore detailed description of the response of ferromagnetic material in weak magnetic fields and the application of Rayleighs law, reference is made'to a paper published in Physics for July .1935, vol.- 6, pp.' 215-226k by Ellwood, entitled 540i the center section Vof the core is provided with two windings, 6 and 8, and 1 and 9, respectively. Windings 6 and 1 Vare vconnected to a source of alternating current .I0

,t 4 Magnetic hysteresis at low flux densities and to a paper published in Journal of Applied Physics for May 1937, vol. 8, pp. 313-358 by Ellwood and Legg entitled Study of magnetic losses at low flux densities in permalloy sheets.

Output terminals 23and 24 areconnected to band-pass filter II. The bandfpass filter II may be of any suitable type and as shown in Fig. 1 is designed to pass a frequency lrange of from 940 cycles to 1060 cycles. This band-pass filter is employed to prevent the higher harmonics and extraneous noisekfrom affecting the operation of the remainder of the system. Band-pass filter I I may be of any suitable type typical examples of whichr are described in detail in United States Patents 1,227,113 and 1,227,114 granted to Campbell onMay 22, 1917, or any ,suitable modication of such filters. The output of the filter I I is connected to an amplifier I2 which is capable of amplifying the frequency-band passed'by filter II. Such amplifiers are well'known and operate in their usual-manner when employed in combinationwith the other elements of the present inv vention. The output of amplier I2 is connected f to thein'put circuit of a mixer or modulating device I3. A source of alternating current I0, which may be 1000 cycles in the specific embodiment shown in thedrawing, is also connected to the mixer I3 through a suitable attenuator I4. The

-attenuator I4 may be of any suitable type and ciples of Radio Communication by Morecroft,

third edition, 1933, on pages 1035 through 1040, also pages 634 through' 646 Vand 765 through 774. For more modern mixer circuits reference is made to chapter 15, pages 99 through 114 of The Radiotron Designers by F. L. Smith.

The output of mixer I3 is connected to two Handbook, third edition,

ters I5 and I6. Filter I5 is a high-pass lter while fil/ter I6 is a low-pass filter. These two filters may be designed and connected together in any suitable manner, as for1 example, as described in greater vdetail in United States Patent 2,076,248 granted April 6, 1937 to Norton which patent' is hereby made a part of the present disclosure a-s if fully included herein. The high-pa'ssiilter I5 is connected to an output amplifier I'I which is in turn connected to anysuitabledetector or receiving :device I8 responsive to the low frequency signallingcurrent. In the exemplary embodiment of the. invention described herein this'low frequency current may be either a `25 or 50-cycle current or both depending upon the bias or operating point vof the core..y Device I8 may include any suitable receiver'such as a headset,sounder, recorder, or printer, as for example,a printer of the type shown Lin' United States Patent 1,904,164

V,granted to S. Morton et al. on Apr1 18, 1933. V

. Low-pass'iilter I6 is designed to pass all frequencies below some cri-ticalY frequencyA which critical frequency is below the desired signalling lfrequency. Assumefor purpose of illustration *thatV filter I6 passes all' frequencies below Y10 cycles. In view of .theifact that the" interferthe output of amplifier I2 .to counter-balance the. earths field.

is alsol encountered a `band-pass flltermay be; em.- .ployed for filter I and a band elimination lter for filter I6 as will be readily understood by per- Sonslskilled in the art. f

.The output of filter I6 is connected through a manually .controlled directcurrent power supply .device 24 to amplifier I9 which. amplifier is a so- .called direct. current amplifier because itv will amplify. continuous aswell as alternating Acurrent .voltages and produce a continuous as well as an alternating current voltagelor vcurrentoutput. output of amplifier. I9 is connected to a phase control circuit 20 which is provided to properly control the phase of the current to be .fed back t0 the windings 25 and 26 on the core sections I and 2 respectively so the entire detecting arrangement will be stable. For a morecomplete description of a phase control equipmentv 20 and the requirements thereof, reference is made to an article entitled Relation between attenuation and phase in feedbackv amplifier design, by H. yW. Bode, inv the Bell .System Technical Journa1,'vol. 19, July 1940, beginning on page 421, which article is hereby made a part of the present application as if fully included herein.

Y As shown in the drawing, the output of the phasecontrol apparatus 2i)v is connected to a manually .controlled direct current power supply. In the exemplary embodiment of this invention shown in thel drawing, themanually controlled power supply comprisesy two batteries 22 or other suitable source of direct current and a manually controlled potentiometer 23. The manually l.controlled power supplies 2| and 24 may be employed Ito control the. bias or operating point: of the core ofthe device.. The manually controlled supply 2| may also be used to relieve the amplifier I9 and the. remainder of thewsystem' ofv alarge part of the. burden cfproviding the yvery low frequency and direct current necessary Usually supply 2|; will be employed to reduce the output required froml amplifiery while supply 24 will be Y ,used to accurately control. the bias or operating point of thefmagnetic materiall of the core structure of the entire device. If' the device isy located aboard. ship and supply 2| is used to reduce the burden on. amplifier I9 it will be necessary to provide a. chart or table or both, showing the position of thee ship.V asv well as its bearing and indicate the proper setting of the manually controlled power supply system. f As: will@ be readily understood by persons -SkillerlA in' the art, the manually controlled supply 2l maybe completely eliminated by providing properly designed amplifier, ters, and phase control circuitsV if it is so desired.

The output of the phase control and manual supply system is connected to' coils 25 and 26 surrounding .the coresections I and 2. respectively amt-aire so.v connected as' to produce a flux which is inopposition'r to and neutralizes the flux produced by the4 external magnetic field applied to thecore structure I, 2 and 3*.,

In operation, the core structure I, 2 and 3 is mountedfinapositionso as to be subjected to the electromagnetic field which itis desired to receive from., When.- an external field is applied to this cores structurel voltages will be induced in leads 23 and 24 which voltages are amplified by amplierl I2 and their dmodulated by mixer 23 so that a voltage relatedto the valueof the applied field is obtained from the output of mixerA I3. yThe high frequencyory signalling component of this vvoltage passes through the highfpass I5 andam- Aplifier I'I to thereceiver I8 while the lowjfreyquency component passesthrough iilter|6,`am-

`plifier I9, manually controlledpower supply`24,

phase control apparatus 20, the kmanuallycontrolled vdirect lcurrent "supply circuit 2| to Windings'26 and 21, where* these currents tend to bali ance ,out the steady and low frequency componentsof the applied field. In this manner.` cores 4 and 5 are in effect subjected to only the signal field orV fields having frequencies of the desired signalling frequency band. Inthis manner itis possible to maintain the total fiux through the cores 4 and 5 to an extremely low value and thus 'provide the maximum 'sensitivity for the device.

In case it is desirable to apply a small biaspto the core structure lit will be` necessary Ito accurately control' the magnitude of the biasso that the total net or effective field applied tothe core .Will not extend beyond the desired region. Furthermore, it is necessary toacCurately :control the operating point to increase the stability and reliabilityofv operation of the device. I l Inorder to apply a bias to the core source 2| or more usuallyk `24 will bef-adjusted to supply a small voltage which will either add to orzoppose the. earths field. 'I'he veffect. of this vvoltage'will bereducedl due to the operation of the feedback circuit. However, a small fraction of this voltage will still be `effective and act as a small bias for the core structure. Under these circumstances the operation. of the. remainder of thecircuits and apparatus rwill be as described above.

Due to the degenerative action of the feedback arrangement the effect of. small changes in the magnitude of sources 2I vand 24 lare reduced so however, will readily appreciate that many otherA types of equipment andA apparatus aresuitable for the various components of the present invention and may be employed therein without material modification. 'I'his inventionis accordingly not limited toany of the specific devices specifically referred to above.

What is claimed is: I I

1. In a. communication system a receiving device comprising an elongated core of ferromagnetic material, a. center portion thereof having a high rate of change of permeability with low val- .ues ofA field; intensity,V meansV for applying a low amplitude. high frequency field to said core,A a receiving amplifier, windings' on said core connected to said amplifier, and- `feed-back;windings also VAupon. saidcore connected to said amplifier for neutralizing the. effects upon. said core of all fields applied thereto. except a field having the same frequency as the signalling frequency of said system and except said high frequency e'ld.

`2. A receiving device responsive to electromagnetic waves. comprising a core' of ferromagnetic material, means for providing a plurality of magneticlpaths through a portion of" said core, apf- 'vpar'atus for applying a low intensity high fre'- 7 quencyA field tof'each ofsad paths in different' directions orphases, loutput windings interlinking saidmagnetic paths and. connected in* Vsuch va manner: that no output voltage is wdeveloped. across said windings when no external 'field is applied Vto saidcore, an amplifierconnected to said out-` put windings, 'feedback windings. also interlink?, the said core, operativeconnections between 'saidA amplifier andsaid' feedback windings for neutralizing the effects of all external fields upon lsaid core and; means rendering said=neutralizing` windings' ineffective. at the signalling .frequency of:said system.

3. In a telegraph system .9, Vreceiving device comprising a long core V.of ferromagnetic -mate-` rial, acenter portion thereof having a, plurality of magnetic paths and composed of ferromagnetic alloy having a high rate of'change of permeability atlow values of field intensity, a source of relatively high. frequency alternating current, windings'interlinking said plurality of paths'con-` nected to said source of high frequency current -forvapplying to said magnetic paths an alternat` fing. current, field of low intensity in the; region `wherethe hysteresis loop of said material is defined by Rayleighs law, output windings interlinking said magnetic paths, a receiving ampli` fierconnected to said'windings, .filter networks -connected to( said amplifienia receiving device connected to said filter networks, and feedback windings interlinking said lcore also connected Vto said network for maintainingy the total field within said magnetic material in Yal1 except lthe signallin'g frequency range and the frequency of said source within thelregionv wherein the hysteresis loss is defined by ltayleighs law.

' 4; In a telegraph system a receiving device comprising a long core of ferromagnetic material, a centerV portion thereto having a plurality of magnetic paths and .composed of ferro-magnetic Vallow having 'a high hysteresis coeicientlfat lowvalues of field intensity,l a source of vrelatively high frequency alternating current, windings interlinking said plurality of paths connected to said source of high frequency current. for applying'to said magnetic paths Van alternatingcur- -rent field of vlow intensity in the regionk where the hysteresisY -loop' of said material is defined by 1Rayleighs law, output windings interlinking said V`magnetic paths, a receiving amplifier connected to said windings, filter networks connected to said amplien a receiving 'device connected to said filter networks, feedback windings 4interlinking said core also connected :to said networks for maintaining the total fieldiwithin said magnetic material in all except the signalling frequency and the frequency of said source range within the region wherein the hysteresis loss is defined by Rayleighs law, and amanually controlled source of direct current connected in said feedback windings for neutralizing the effect of the constant portion of earths magnetic field upon said core.

5. Apparatus responsiveV to electromagnetic fields comprising a core structure including end sections and a center.section,'a plurality of magnetic paths extending between each of said end Ysections through said center section, alternating current power supply windings interlinking said paths through said center section, a source of relatively high frequency alternating current', interconnections betweenv said ysource and said windings for supplying alrelatively high fre--v quency alternating current field vof low intensity to each of said plurality of paths ina different ,phase relation, outputj windings also interlink;-

ingthe plurality of said pathsand connected.A so

' that nov output 'potential differenceis'developed across. the output terminals: thereof. ,whenn'o exyternall'field is applied tojsaid core; an amplifier connected to' saidoutput terminal, ya mixing or modulating device; connected to 'the .output .of said amplifier and to said highf'requencyr :source -for developing the difference between' putvoltagefand the frequency of said high-fre.-

quency current, filter equipment connected to the Y output ofsaid mixing equipment for` selectingthe magnetic fields comprising a core of ferromafgf signal frequency 5 thereof, :feedback windings also interlinking said core, connected'equipmentffor connecting said feedbackV winding tosaid feednal field applied to said core, a mixer or ,demodulator connected Vto said outputwinding 'terminals and to Vsaid sourceofhigh frequency alternating current for deriving an output havingV a wave form substantially the samev as the wave' form .of the applied'external magnetic eld.

7..,Asignal receiving device,..comprising a.core of ferromagnetic material, alternating current power supply windings interlinking said core, 'out'- put windings interlinkingv said core Aand connected to` develop an output which isfjointly afunction of saidpower supply alternating current and all other magnetic elds to ,which saidcore is "subjected, a mixer or demodulator .connected to said output windings and provided with a power input terminal for deriving an output having a'wave form substantially the same* as the .wave form'of f the magnetic field to whichsaid core is subjected vexclusive of the field created by said power sup'- ply windings, feedback windings 'interlinking `said core, filter and phase control networksconnected between said mixer andY said feedback windings Vfor re'ducingfthe field to which said core is sub'- jected except for specified frequencies or frequency ranges. .f fp l Y 8.- A magnetic device responsive to external vnetic material, a source of Valternating current having a frequency which is high compared tothe frequencies of said external vfield'torwhich said ,device is responsive, .power windings interlink-v Y ingv said core, outputrwi'ndings also'interlinking -said core for developing ayoltage which is a func'- tion` of the external field rappliedrto. said core, feedback windings also interlinking's'aid"core,` a Yfeedback circu'itsextendingv from'said output tei"- minalsand including said feedbackwindings'to supply current to said feedbackwindingsV to neutralize a portionof the effect of the externalfield applied. to, Vsaid'core, a directA current., source' of 'potentialconnected in said feedback-circuit'to rapplyya bias 'to' saidY core through said feedback windings;v` p .Y Y f a voltage having afrequencyiwhichis the 'frequency 1of.- saidzout- 

